Heater combustion control system



May 13, 1952 F. A. RYDER ErAL HEATER COMBUSTION CONTROL SYSTEM Filed May19, 1949 VNN .NSEM ik FNN mana/n W vIII'- il lHI Patented May 13, 1952UNITED STAT ES PATENT OFF I CE A 2,596,290 HEATER CoM'EUsTIoN 'CONTROLSYSTEM -Frank. Ryder'nd Donald A. Potter, Indianapolis, Ind., assignorsto StewartrWarner Corporation, Chicago, Ill.,-a corporationof VirginiaApplicatioiiMay 19, 1949, serial No. 94,095

31Claiins. 1

The present invention relates to aircraft heater combustion controlsystems primarily,` although certain aspects of the invention may beapplied toV heaters of more `general application.

Y `Recent,trendsiri aircraft vheater designindi'eate the desirability ofheaters which burn the aircraft vengine fuel,usually "gasoline, in asealed chamber which comprises the 'combustion chamber and the heatAexchanger of the heater. This sealed chamber 'has inlet rand outletopenings which terminate outside the aircraft. Ventilating airto beheated is passed in contact 'with the heat exchanger on its way to the4space to be heated. Usual1y, `air 'both for 'eoiiibu'stion and for'ventila- 'tion is Obtained from 4rams or 'scoops which frefquentlyfcomprise openingsvin the leadingfedge of the aircraft. wing.Although heaters jof this 'general type are largely used foraircraftcabin heating they are finding increased usefulness in heating the`wings 'and tailsurfaces of aircraft, to prevent the formation of ice.purpose must have enormous heatingcapacity by prdinfay standards eventhough their size and Weight must'be kept to a minimum. Most importantof all, theymust be inherentlysafe and rreliable. Reliabilityisespe'ciallyimportant when theheaters are used for anti-icing purposes. A

'heater of this general character isshown in application 4"serial No.l8,106 filed by Gera1d .Ja-cots l February'l33 1948, riow'Patent No."2;53193g9, dated November 28, 1950, for Intermittent Cofmbus'- tionSpark Ignition Aircraft Heate'rf o o In `order'to achieve lowweight,great'safety,

,anda highv degree of' reliability it is advisable that" the heatertyznitrol'r system be comprised largely of componentswhose expected lifecan `be quite well predicted. `It' lis an `inherent advantagetherefore,`to Vuse equipment for control purpqses which isas little coj'mpucated-agpossible, consideringthe objective to be-obtained; and; in addition,whereever possible it is advantageous "to use elements in the controlsystem Vwhich have been widely manufactured nfor other purposes, sincethe past usage ofJ suchelernents in other applications has in Amostinstances resultedin the design beinglstan'dardized upon abasis whichinsures predictable performance.

yA `heater of this type should beso arranged that; it may be turned onand olf easilyat a iemtepoint and should te automaticallycontoued so.,that if it eoesriot ,start within a reasonable smeerif ai aijiy einenlstepsoperation for an excessive iqeri'odp'rv if` the heater'overheatsfor any reason the fuel to the heaterv'vill beturned off. Thisis especallyirportat.when heaters Heaters for 'this therefore,- is high.In addition to the desirable featureshjust recited which have to do withsafety `of operation of the heater it is necessarythat ltheheatings'ystem automatically supply air at the proper temperature toaccomplish the. desired purpose. Todo this may require modulating theheat outp'ut of the heater all the way from full output down to anoutput soA low that eflicient continuous operation of the heater isimpractical either because combustion may fail or because iciency may beexcessivelyl low.

lIn` yie'w of the above it is `the principal object of our invention toprovide acontrol system for a spark ignited liquid fuel burningcombustion heater which accomplishes all of the above set out objectivesin an extremely simple manner and with equipment which is known to bereliable in operation.

Still another object of our invention is to provide a heater controlsystem which obtains great simplicity by depending for itsk operation toa considerable extent upon the inherent time lag characteristics ofcertain control elements and which co-ordinates these time lagcharacteristics with a-timirlg mechanism matched to their par- AVticularcharacteristics.

Other objects and advantages will become apparent from thefollowingdescription of a preferr'ed embodiment of our invention which isillustiatedin the accompanying drawing.

-Thesi'n'gle-gurein the drawings is a diagrammatic representation of anaircraft heater and a control systeml thereof which embodies theteachings of the present invention.

Referring to the drawing, an aircraft heater is indicated generally bythe numeral 10,. This heater receives Ventilating air to be heated ibywayof a duct I2 and delivers the heated air through a duct `Hl connectedat the oppositeend of the heater. Air for combustion is supplied to theheater through a conduit I6 'whilethe heater exhaust is passed overboardthrough an yexhaust f pipe Il.V In normal usethe duct |2- will beconnected to a scoop or ram located` intheileajding edge ofv the wing orat some other position on `the outside of the aircraft. The airsuppliedto the combustion air conduit I6 preferably willbe obtained froma separate and smaller scoop although if desired the same scoopAmayfsupply air to both the conduit Hand the duct- I2.

Fuel is supplied to the heater through a line i8 which usually leadsfrom the engine fuel supply system by way of a pipe 2!) and thencethrough a safety shut-oil" valve 22; This valve in turn is connected bya pipe 24 toa second shut-off valve 25..,which in turnfconncts with thereferred to line is. Bctnvalves. 22 andataie electromagnetieally yoperated and' are so arranged: that 3 when de-energized the valves willbe closed but will permit fuel flow therethrough when electricallyenergized by means of the circuit to be described presently.

Flow of Ventilating air through the supply duct l2 is controlled by adamper therein indimaintain them in this position so long .as enerrgized. When the motor is de-energized the dampers 28 and 30 return tothe closed position, Since such motors have wide application incontrolling the airflow through ducts in aircraft systems generally nodetailed description of this motor need be given since it is a wellunderstood control component familiar to those skilled in the art.

The control circuit shown receives energy from the aircraft battery 3d,one side of which is the master switch is connected to a main lead'40which connectsin turn to the damper motor 32. Thus, closing the masterswitch v38causes energization of damper motor 32, thereby opening .thedampers 28 and 36 so as to permit the passage of Ventilating air throughthe system. This conditions the system for starting or if de- Sired,closure of the switch 38 alone permits the flow of 'Ventilating air forgeneral Ventilating purposes inV an unheated condition.

VThe line 40 is also connected through a single pole, single throwheater switch 42 to a thermostatic overheat switch 44 which perferablyis of the manually reset type. This overheat switch in turn is connectedby a line 46 to the magnetic coil 48 ofthe magnetic fuel vvalve 22. Theoverbut opens to shut off the heater whenever the 4 if desired. The cam58 is so shaped that when it is in the position shown the switch v60 isopen. As soon as the motory 56.has started, however, and has operatedfor a few seconds the cam 58 will rotate to a position where the switch66 is closed and the cam will maintain the switch in closed positionuntil it has rotated 360 degrees whereupon the switch 60 will reopen.The portemperature of thev Ventilating airleaving `the heater becomesexcessive. Its mounting position relative to theheater is indicated by asmall circle on the hot air duct I4 at 54a.

Line 40 is also connected by lead 56 to `one of the Contacts of amomentary contact push button 52, the other contact of which isconnected by a lead 54 to a one revolution motor 56. The shaft ofthismotor is geared down so as to have a speed of approximately onerevolution per minute and this low speed shaft is connected to drive acam 58 which operates a switch 66 once per revolutionI One terminal ofthis switch 66 is connected by a lead 62 to the motor side of the pushbutton switch 52 and also to the coil 64 of a normally open relay 66.The other terminal of the switch 6D isl connected by a line 66 to themain lead 46.

Although various types of motors may be used in the applicationindicated at 56 we prefer to use what is commonly known as a reedsychronized motor. These motors are small and are operated on directcurrent and have a constant speed which is determined by the frequencyof a vibrating reed. Of course, any other motor having comparativelyconstant speed characteristics may be used in place of a reedsynchronized motor tion of the circuit just described, therefore, issuch that closure of the switch 66 by the motor 56 and cam 58 energizesthe coil 64 for one complete revolution of the motor 56, or in otherwords, for approximately one minute. As soon as the minute has passedthe switch 66 reopens, thereby de-energizing the coil 64 and permittingthe relay 66 to fall openl (the relay 'being of the normally open,single pole, single throw type). Push button 52 when momentarilydepressed .energizes the motor 56 directly fromthe line 4U and places itin operation and simultaneously by way of the line62 energizes the relaycoil 6d. A few seconds thereafter the holding circuit by way of the line68 and switch 66 will have kbeen closed so that release VofV the pushbutton 52 does not de-energize either the motor 56 or the coil 64, bothof these elements remaining energized for approximately one minute.

One of the contacts 10 of the normally open relay switch 66 isVconnected to the leadL 46 so that this contact -is energizedsimultaneously with the energization of the coil V118 of valve 22. Theother contact 'l2 of relay switch 66 is connected to a normally closedthermostatic cycling switch 'I4 by a lead 16. This thermostatic cyclingswitch` isof the normally closed type and is located intheheater'ven'tilating air outlet duct I4 at a point indicated by thenumeral 14a. This switch is of the quick response type. That is, itfollows fluctuations in the temperature of the Ventilating air quiterapidly and has a comparatively small differential (a matter of two orthree degrees or so) between its on and olf positions. The thermostaticswitch in turn is connected to the magnetic coil 18 of theelectromagnetic valve 26 which for convenience will be known as thecycling valve. c Y

A normally open flame detector/thermostatic switch indicated by thenumeral 80, is wired in parallel with the relay switch 66. This normallyopen flame detector switch is conveniently installed in the exhaust pipel1 at the point indicated by the numeral a and has characteristicsunlike the thermostatic cycling switch 14. It is arranged lso that itdoes notclose until a temperature of, for instance, 700 degrees F.or sois reached, but once closed it does not reopen until the temperature hasdropped to, for instance, 200 degrees F. or'so.V This operation maybeaccomplished by using'a snap action switch operated by an expanding andc ontracting element. Such switches are well known. A typical example isshown in application Serial No. 684,350, filed July 17, 1946, by ArthurR. Collins, Frank A. Ryder, j and Ross D. Randall, now VPatent No.2,581,942, dated January 8, 1952. In. that example a simple snap actionswitch is operated by the differential expansion of a fused quartz rodand a stainless steel tube.v The on-ofl' temperature differential of theswitch may be set as desired by properly proportioning the length of thedifferentially expanding elements with the switch operatingcharacteristics.

` A lead 82 is connected to the line 16 yand to a vibratory reed-likeelement which 4moves under 'the influence `of' an electromagnetic coil88. IThis coil is connected to a stationary contact 90 which engagescontact 8 6 when the vibratory element indicated ,generally by thenumeral 92 is in its ;free,position. The other-movable contact 84 isvassociated with a stationary contact 04 which is in 'engagementtherewith when the `vibratory element 02 is` in its free vposition andwhich is con- .nectedby a lead Sfte/one end Aof the primary 98 of astep-up `ignition transformer |00. The other :end of the primary isgrounded while Athe secondary |02 .is vconnected `to a spark plug |04`disposed with its ,sparking end within the burner of the heater I0.

When the line. 82 is energized, therefore, current flows through thecontacts 86 andy90 to the .coil 88, thereby energizing Athis coil andvmoving the vibratory 'element 82 so as to separate contacts 86 and 00.The coil 88 'is `therefore deenergized and permits contact '88 to bebrought lonce more against contact v90, thereby repeating to the primary98 isvstepped up in the secondary |02 and energizes the spark plug |04at a Voltage which insures continuous ignition so long as the lead 82 isenergized.

The control system as a unit operates in the following manner. Theheater is conditioned for operation by closing the master switch 38.This energizes the vdamper motor 32 and opens the dampers 28 and 30,therebysupplying combustion air and Ventilating air to the heater l0.Subsequently, when the heater -switch 42 is closed, the magnetic valve22 will be energized and opened. This valve normally will be located ata remote point in the fuel system so as to be comparatively close eitherto a special pump which supplies fuel to the heater or to the aircraftengine fuel line. If a separate heater fuel pump is used, it may replacevalve 22 or be wired inparallel with coil 48. Opening of the valve 22,however, does not startl the heater but simply supplies fuel to thebranch 'line leading to the heater.

The heater is now started by depressing the push button 52 and holdingit for a matter of three seconds or so, thereby energizing the onerevolution motor 58 and `relay coil 84. As previously explained, afterthe one revolution motor 56 has been energized by the push button 52 foralmatter of three seconds or so the holding circuit through the lead 68comes into operation due to the closure of the 'switch 60 by the cam 5B.

As soon as the relay 84 is energized the contacts` and 12 are closed,thereby energizing the ignition system lead 82 and the cycling switchlead 16. sparking at the spark plug |04 therefore immediately takesplace and'since the thermostat I4 is `cold and in the closed positionthe coil 18 will be energized, thereby opening the cycling valve 26. Theheater now is receiving Ventilating air, combustion air, and fuel, andsparking is takingvplace at the igniterplug |04. As soon as fuel reachesthe nozzle in the heater, therefore;

ignition takes place almost instantaneously.

Because heaters of the type used in aircraft `applications havecomparatively low massin their heat exchangers in proportion to the heatoutput ofthe heater, the temperature of the Ventilating 6 air will riserapidly as will the temperature `of :the exhaust flowing through thepipe H. Because the thermostatic switch 8 0 does Vnot close .until aten'iperatu-rey of 700 degrees F. or 'so is reached, there :will be a'considerable time interval` during which it remains open during thestarting cycle. It does, however, provided `the heater is functioningproperly, close withiirless than a minute and shunt the contacts T0 and12 before the camg .has completed its single revolution which normallytakes place in about one minute. As soon as the cam 58 vcompletes itsrevolution the motor -56 Ais cle-energized as is the relay coil @4,thereby permitting the contacts 18 and l2 to be separated. This,however, does not affect operation of the heater ifv the thermostaticflame detector 8 0 has closed inthe meantime.

On the other hand, if the heater does not start properly, the thermostatS0 will not have reached a temperature of '700 degrees meanwhile, withthe result that as soon as the cam 58 completes its single revolution`so `as to open the relay contacts 86 the cycling valve coil 18 will bede-energized, thereby turning oir the fuel to the heater.

Also, the ignition system connected tothe line 82 will be cle-energized.The heater therefore is placed out of operation unless vit startslpromptly and operates long enough so that its exhaust temperature risesto *700 degrees or 'so within less than one minute.

If the heater functions normally the flame detector switch remainsclosed andA fuel continues to be supplied to the heater untilthetemperature of the cycling switch 'I4 rises to the set level. Whenthis happens, the switch 'I4 opens, thereby cle-energizing the coil 1.8`and `causing the cycling valve 26 to close. This shuts off theflow offuel to the heater, thereby extinguishingv combustion. The ignitionsystem, however, continues to function so that if the thermostat E4cools s uficiently to reclose before the flame detector switch 88 hascooled to 200 degrees F. (its opening point) the heater will besubstantially instantaneously restarted by reopening` of the valve 20.

Since the cycling characteristics of the cycling switch 'E4 are suchthat a Variation in its temperature of a matter of a feW degrees one wayor the other will result in its opening or closing, the heating systemwill continue to operatev indenitely, cycling the combustion on and offwithout the temperature of the Aflame detector switch ever droppingbelow 200 degrees. If, however, the heater cycles to the off positionand then does not relight, or if for any other reason combustion failsor the overheat switch '44 op erates, the temperature of the flamedetector switch S0 will presently drop below 200| degrees, therebycausing the switch il'to open with the result that the ignition systemand the lead 16 areA permanently rie-energized until the push button 52is pressed to start the heater operating all over again.

Because of the low mass of the heat exchanger, consideringthe heatoutput achieved therefrom, the temperature of the metal of the` heatexchanger and therefore the temperatureV of the Ventilating air willquite promptly follow the combustion conditions in the burner. In otherwords, if combustion is extinguished,` the temperature of theVentilating air Will drop enough quite promptly to recloseswitch 'i4since the cycling thermostat is arranged to open orclose upon a smalldifferential. The rapidcylingof the thermostat 74 also maintains thetemperature of 7 the Ventilating air substantially constant, Forinstance, it may be desired to maintain the temperature of theVentilating air at approximately l25 degrees. As the ytemperature of theventilating air rises to 125 degrees the thermostat opens.

The temperature of the Ventilating air will, of course, overshootslightly beyond this point before starting to drop. As it drops to 125degrees or so the cycling switch will again close and although theburner lights almost instantly the' temperature of the Ventilating airwill overshoot slightly below 125 degrees before beginning to riseagain. Theoff cycles, however, are of such Yshortrduration that thetemperature of the air remains substantially constant for all practicalpurposes and the name detector switch which has a large differentialwill never open unless the burner has been off for a period longer thanwould be normal even though a relatively low temperature of theVentilating air happened to be required.

It will be seen from the above that through the use of a flame detectorwith a wide differential and slow response in conjunction with a cyclingswitch of rapid response used together with the one Yminute holdingcircuit provided by the one revolution motor and a heat exchanger of lowmass and high output, it is possible throughV the use of a comparativelysimple circuit and welll tested circuit components to provide all of thecontrol features required by an aircraft heating system even though thetemperature of the vensaid second temperature .sensitive switchingmeans.

u 2. A heating system comprising'a fuel burning air heater, electricenergy responsive'means to supply fuel to the heater when energized, anelectric circuit for Venergizing ysaid fuelY supplying means,temperature sensitive Switch means in said circuit for interrupting thecircuit for an timed interval longer than the first said intervaltilating air is required to be maintained at a l Vtrolling the flowthrough said connections, electric energy responsive means to open saiddampers when energized and to close said dampers when deeneigized, anelectric circuit including an onoff switch for energizing said electricenergy responsive means, electric energy responsive means to supply fuelto the heater when energized, a second electric circuit for energizingsaid fuel supplying means, said second circuit being connected toreceive energy through said first circuit so that said second circuit isdeenergized unless said rst circuit is energized, temperature sensitiveswitch means in said second circuit for interrupting the second circuitfor an interval until the temperature of said heater has reached apredetermined level indicating proper operation of the heater and forcompleting said second circuit when said predetermined level is reached,other switching means connected in parallel with' the last said switchmeans, and means for closing said other switching means for a singletimed interval longer than the irstA said interval at the start oflheater operation, and a second temperature sensitive switching means insaid second circuit for completing or interrupting said second circuitas the temperature of the heated air drops below or rises above adesired level, the temperature differential of operation of said firsttemperature sensitive switch means being several times as large as thetemperature differential of at the start of heater operatiornand asecond temperature sensitive switching 'means in said circuit forcompleting or interrupting said 4circuit as the temperature; of theheated air drops below orrises abovea desired level, the temperaturedifferential of operation of said rst temperature sensitive switch'meansbeing Vseveral times aslarge as the temperaturediiferential of saidsecond temperature sensitive switching .3. A heating system comprising avfuel burning air heater having a `Ventilating air connection, meansfor'controlling the iiow through said connection, electric energyresponsive meansto operate the controlling means to open said connectionwhen energized and to close said connection when deenergized, anelectric circuit including an on-of switch for energizing ,said electricenergy responsive means, electric energy responsive meansto supply fuelto the heater when encompleting said second Ycircuit when saidpredetermined level is reached, other switching means connected inparallel withrthe last said switch means, and means for closing saidotherswitching means for a single timed interval longer than the rstsaid interval at the start of heater operation, and a second temperaturesensitive switching means in said secondA circuit for completing orinterrupting said second circuit as the temperature of vthe heated airdrops below or rises `above a desired level, said rst temperaturesensitive switch having an .operating differential several-times aslarge as said second temperature sensitive switch. l

. FRANK A. RYDER.'

DONALD A. POTTER.

REFERENCES CITED The following references are of record in the file Vofthis patent:

'UNITED STATES PATENTS Nmnber Name Y -Date 1,270,877 Schulz July-2, 19182,160,592 Y King Y May 30, 1939 2,230,446 Baker Feb. 4, 1941 2,428,867DeLancey Oct. 14, 1947 2,451,760 McCollum Oct. 19, 1948 2,484,602 AubertOct. 11, 1949

